This invention relates to the art of encoding electrical signals for transmission over a long cable; and more particularly, it relates to the art of frequency-encoding digital signals in a manner which reduces their distortion at the receiving end of the cable.
One system in which such frequency-encoded digital signals are transmitted is illustrated in FIG. 1. This system includes a device 10 having an encoder 11 that receives digital signals on an input terminal 12 and converts them to frequency-encoded signals on an output terminal 13. A transmitter 14 receives the frequency-encoded signals on an input terminal 15 and converts them to a pair of differential frequency-encoded signals on a pair of output terminal 16a and 16b.
Terminals 16a and 16b couple to one end of a long cable 20 which is comprised of a pair of twisted wires 21a and 21b. A suitable electromagnetic shield may also be provided as an enclosure for the pair of twisted wires. wires 21a and 21b then couple at their opposite end to a pair of input terminals 31a and 31b of a receiver 32. Receiver 32 operates to convert the differential pair of signals on its input terminals to a corresponding digital signal on its output terminal 33.
A problem, however, with the above-described system of FIG. 1 is that the long cable 20 distorts the signals from transmitter 14, which in turn can cause errors at the receiver 32. To understand this problem, consider first the waveforms of FIGS. 2A and 2B. FIG. 2A shows the digital signal DS on input terminal 12 of encoder 11; and FIG. 2B shows a conventional frequency-encoded signal FES on input terminal 15 of transmitter 14. Signal FES includes a low frequency component f.sub.L which represents a digital "1", and it also includes a high frequency component f.sub.H which represents a digital "0". Every half-cycle of the low frequency component is of a single time duration (1/2f.sub.L) and every half-cycle in the high frequency is of another signal time duration (1/2f.sub.H).
Next, consider the waveform of FIG. 2C. It shows a signal V.sub.R which is the differential voltage across the input terminals of receiver 32. Ideally, signal V.sub.R should have the same shape as signal FES. However, a comparison of FIG. 2B with FIG. 2C shows that signal V.sub.R is distorted following the change from the low frequency to the high frequency. Reference numeral 41 indicates the point in time at which this distortion (hereinafter referred to as transmission line distortion) begins.
Due to the transmission line distortion in signal V.sub.R, the envelope 42 of the high frequency component of signal V.sub.R is not flat. Instead, the portion of the envelope 42 which follows immediately after the low-high frequency transition 41 is bent toward the peak voltage of the last low frequency half-cycle that preceded the transition. In FIG. 2C, envelope 42 bends in a negative direction because the peak voltage in the last low frequency half-cycle that preceded transition 41 was negative. Conversely, in FIG. 2D, the envelope 42 bends in a positive direction because the peak voltage of the last low frequency half-cycle that preceded transition 41 was positive.
Since the above-described bending of envelope 42 occurs, the magnitude of the peak voltage of the first high frequency half-cycle that follows transition 41 is too small; and further, the magnitude of the next high frequency half-cycle is too large. Also, these distortions in the magnitude of the half-cycle peak voltages become more pronounced as the length of cable 20 increases.Thus, as the length of cable 20 is increased, a point is eventually reached at which receiver 32 cannot correctly convert the frequency-encoded data on its input terminals back to the corresponding digital data.